rcu: Add a TINY_PREEMPT_RCU

Implement a small-memory-footprint uniprocessor-only implementation of
preemptible RCU.  This implementation uses but a single blocked-tasks
list rather than the combinatorial number used per leaf rcu_node by
TREE_PREEMPT_RCU, which reduces memory consumption and greatly simplifies
processing.  This version also takes advantage of uniprocessor execution
to accelerate grace periods in the case where there are no readers.

The general design is otherwise broadly similar to that of TREE_PREEMPT_RCU.

This implementation is a step towards having RCU implementation driven
off of the SMP and PREEMPT kernel configuration variables, which can
happen once this implementation has accumulated sufficient experience.

Removed ACCESS_ONCE() from __rcu_read_unlock() and added barrier() as
suggested by Steve Rostedt in order to avoid the compiler-reordering
issue noted by Mathieu Desnoyers (http://lkml.org/lkml/2010/8/16/183).

As can be seen below, CONFIG_TINY_PREEMPT_RCU represents almost 5Kbyte
savings compared to CONFIG_TREE_PREEMPT_RCU.  Of course, for non-real-time
workloads, CONFIG_TINY_RCU is even better.

	CONFIG_TREE_PREEMPT_RCU

	   text	   data	    bss	    dec	   filename
	     13	      0	      0	     13	   kernel/rcupdate.o
	   6170	    825	     28	   7023	   kernel/rcutree.o
				   ----
				   7026    Total

	CONFIG_TINY_PREEMPT_RCU

	   text	   data	    bss	    dec	   filename
	     13	      0	      0	     13	   kernel/rcupdate.o
	   2081	     81	      8	   2170	   kernel/rcutiny.o
				   ----
				   2183    Total

	CONFIG_TINY_RCU (non-preemptible)

	   text	   data	    bss	    dec	   filename
	     13	      0	      0	     13	   kernel/rcupdate.o
	    719	     25	      0	    744	   kernel/rcutiny.o
				    ---
				    757    Total

Requested-by: Loïc Minier <loic.minier@canonical.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 15 insertions, 1 deletions

diff --git a/init/Kconfig b/init/Kconfig
index dbc08baad77e..a619a1ac7f4c 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -348,7 +348,7 @@ config TREE_RCU
           smaller systems.
 
 config TREE_PREEMPT_RCU
-        bool "Preemptable tree-based hierarchical RCU"
+        bool "Preemptible tree-based hierarchical RCU"
         depends on PREEMPT
         help
           This option selects the RCU implementation that is
@@ -366,8 +366,22 @@ config TINY_RCU
           is not required.  This option greatly reduces the
           memory footprint of RCU.
 
+config TINY_PREEMPT_RCU
+        bool "Preemptible UP-only small-memory-footprint RCU"
+        depends on !SMP && PREEMPT
+        help
+          This option selects the RCU implementation that is designed
+          for real-time UP systems.  This option greatly reduces the
+          memory footprint of RCU.
+
 endchoice
 
+config PREEMPT_RCU
+        def_bool ( TREE_PREEMPT_RCU || TINY_PREEMPT_RCU )
+        help
+          This option enables preemptible-RCU code that is common between
+          the TREE_PREEMPT_RCU and TINY_PREEMPT_RCU implementations.
+
 config RCU_TRACE
         bool "Enable tracing for RCU"
         depends on TREE_RCU || TREE_PREEMPT_RCU